Because typical LCD devices have the advantages of portability, low power consumption, and low radiation, they have been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras, and the like. Furthermore, LCD devices are considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.
In FIG. 3, a conventional normal-white type LCD 10 includes an LCD panel 100 and a backlight module 110 facing the LCD panel 100. The backlight module 110 is configured to illuminate the LCD panel 100 with even planar light.
The LCD panel 100 is a normal-white type LCD panel, and includes a first substrate 101, a second substrate 102 facing the first substrate 101, a liquid crystal layer 103 sandwiched between the two substrates 101 and 102, and a first polarizer film 104 and a second polarizer film 105 respectively disposed at outer sides of the two substrates 101 and 102. Polarization axes of the first polarizer film 104 and the second polarizer film 105 are perpendicular.
The backlight module 110 includes a light source 111, a light guide plate 112, and a reflecting film 113. The light guide plate 112 includes a light incident surface 1120, a top light emitting surface 1121 adjacent to the light incident surface 1120, and a bottom surface 1122. The light source 111 is located at a side of the light incident surface 1120 and configured to emit polarized light “c”. The reflecting film 113 is located at a side of the bottom surface 1122 to reflect light from the bottom surface 1122 back to the light guide plate 112.
After the polarized light emitted by the light source 111 reaches the light guide plate 112, some of the polarized light is emitted directly by the light emitting surface 1121 of the light guide plate 112 and reaches the second polarizer film 105. Other polarized light is emitted by the bottom surface 1122, and is then reflected by the reflecting film 113 back to the light guide plate 112. Finally, the other polarized light reaching the light guide plate 112 is emitted from the light emitting surface 1121 thereof and reaches the second polarizer film 105.
Some of the light reaching the second polarizer film 105 with a polarization axis parallel to a polarization axis “b” of the second polarizer film 105 (hereinafter, “first polarized light”) can pass through the second polarizer film 105 to illuminate the LCD panel 100 (see below).
In detail, when driving voltages are not applied to the normal-white type LCD 10, the polarization axis of the first polarized light is rotated by 90° to become parallel with a polarization axis “a” of the first polarizer film 104 when the first polarized light passes through the liquid crystal layer 103. Thus the rotated first polarized light having the polarization axis parallel to the polarization axis “a” of the first polarizer film 104 can pass the first polarizer film 104 so that the LCD panel 100 emits light and can display images.
When driving voltages are applied to the normal-white type LCD 10, the liquid crystal molecules of the liquid crystal layer 103 twist to become oriented approximately perpendicular to the two substrates 101, 102 and thus the polarization axis of the first polarized light cannot be rotated by the liquid crystal layer 103. Understandably, the first polarized light reaching the first polarizer film 104 has the polarization axis perpendicular to the polarization axis “a” of the first polarizer film 104 and therefore cannot pass the first polarizer film 104. Thus the LCD panel 100 does not emit light and has no image.
Normally, because the polarization axis of the first polarized light is not fully parallel to the polarization axis “b” of the second polarizer film 105, only some (or most) of the first polarized light can pass through the second polarizer film 105 to eventually emit from the LCD panel 100, while the other first polarized light having polarization axes perpendicular to the polarization axis “b” of the second polarizer film 105 is absorbed by the second polarizer film 105. This may considerably reduce the operating efficiency of the normal-white type LCD 10.
What is called for, thus, is a backlight module overcoming the limitations described, and an LCD utilizing such a backlight module.